The present invention relates to photovoltaic devices containing amorphous silicon and having improved short wavelength photoresponse. More particularly, the present invention relates to thin-film amorphous silicon p-i-n devices having reduced p-dopant contamination of the i-layer neat the p/i interface and their method of manufacture.
Photovoltaic devices such as, for example, solar cells and photodetectors are capable of converting solar radiation into usable electrical energy. These devices may be fabricated by sandwiching certain semiconductor materials between two electrical contacts. As disclosed in U.S. Pat. No. 4,064,521, which is incorporated herein by reference to the extent necessary to effect a thorough understanding of the background of the present invention, one semiconductor material that can be used is a body of amorphous silicon deposited by glow discharge in silane. Photovoltaic devices utilizing amorphous silicon typically contain one or more p-i-n or inverted p-i-n junctions.
The photoresponse of such thin-film amorphous silicon p-i-n devices to light whose wavelength lies between 400-560 nanometers is less effficient than expected from measurements of the optical absorption of the overlying layers. The short wavelength, i.e., "blue," response of the device may be low for several reasons: the electric field at the p/i interface may be weak, slowing carrier transport and permitting more carriers to recombine; the electron or hole lifetime may be reduced at the front of the device due to contamination; and interface recombination at the p/i interface may remove carriers and prevent their collection.
It has been experimentally observed that the presence of a p-type dopant such as boron in amorphous silicon devices and in the amorphous silicon deposition system lowers blue response. Tests were conducted on amorphous silicon p-i-n devices utilizing a silicon carbide p-layer fabricated in a single glow discharge deposition chamber batch system. Such devices were prepared using a 50/50 mixture of silane and methane for the p-layers. The only variable in the experiment was the concentration of diborane used in the doping of the p-layer, which varied between approximately 0.1% to 0.3%. I have found that the blue response of the devices decreased monotonically with increasing boron content of the p-layer. Measurements of the quantum efficiency as a function of electrical bias confirmed a reduction in the effective collection length at short wavelengths; the collection length at longer wavelengths was affected to a lesser degree.